The present invention relates to a receiving circuit of a television receiver for receiving an FM-modulated television signal.
A new television broadcasting system for transmitting television signals by use of an artificial satellite is contemplated. In this system, the FM-modulated television signal is transmitted on an electromagnetic wave of SHF band, especially, between 11 GHz and 12 GHz. A block diagram of a conventional receiving circuit for receiving the FM television signal is shown in FIG. 1. This receiving circuit receives the FM television signal of SHF band transmitted from the artificial satellite and converts it into a video signal. The FM television signal of approximately 12 GHz in frequency is received at an antenna 1 and applied to a first mixer 2 connected to the antenna 1. The first mixer 2 is connected to a first local oscillator 3 for producing a local oscillation signal which is supplied to the first mixer 2. The frequency of this local oscillation signal is confined to a predetermined value such as 10.74 GHz. The FM television signal is converted to a signal of UHF band of about 960 to 1460 MHz by the first mixer 2 and the converted signal is applied to an amplifier 4 connected to the first mixer 2. Generally, the antenna 1, the first mixer 2, the first local oscillator 3 and the amplifier 4 are installed outdoors. The first mixer 2, the first local oscillator 3 and the amplifier 4 make up a first heterodyne receiving circuit, by which the television signal is converted to a UHF signal and transmitted indoors. The second mixer 6 is connected with a second local oscillator 8 for generating a local oscillation signal to be applied to the second mixer 6. The frequency of this local oscillation signal is varied in accordance with a channel to be received, e.g. in accordance with a tuning voltage generated by a channel selector 7. The television signal in UHF band is converted into a signal of VHF band of about 130 MHz at a second heterodyne receiving circuit including the second mixer 6 and the second local oscillator 8. The signal thus converted is applied to a limiter circuit 9 connected to the second mixer 6. At the limiter circuit 9, an amplitude of the VHF signal is limited to remove AM noise. An output signal of the limiter circuit 9 is applied to an FM detector 10, where the signal is FM-detected to be converted into a video signal and an audio signal. The FM-detected signal flows into two routes. One of the routes contains an audio signal demodulator circuit 11 where the FM-detected signal is demodulated into an audio signal. The other route contains an amplifier 12 where it is amplified. The amplified signal is applied to an emphasis circuit where it is deemphasized, and then applied to a clamp circuit 14 where an energy dispersal signal contained therein is removed. An output signal of the clamp circuit 14 passes through an amplifier 15 and goes out as a video television signal. The amplifier 15 is followed by a video signal output circuit and a picture tube for reproducing an original picture. Heretofore, the energy dispersal signal has been removed by the clamp circuit 14.
FIG. 2 illustrates a television signal with the energy dispersal signal 16 superposed thereon. Specifically, the energy dispersal signal 16 is a triangle wave signal at frequency which is 1/n (n is an integer) of a field frequency of the video signal. In connection with the frequency of the energy dispersal signal, if n=2, it is 1/2 of the field frequency, i.e. 30 Hz. The energy dispersal signal is superposed on the television signal in synchronism with a vertical synchronizing signal 18 of the video signal. A conventional clamp circuit is illustrated in FIG. 3. This clamp circuit is a diode clamp circuit composed of a capacitor 20 and a diode 21. The diode 21 is connected to a voltage source of V.sub.0 composed of resistors 22 and 23 and a capacitor 24. With this circuitry, when the signal as shown in FIG. 2 is input to an input terminal 19, a signal waveform at an output terminal 25 changes with a capacitance of the capacitor 20 in the clamp circuit. If the capacitance of the capacitor 20 is selected to be large, the charge/discharge time constant of the capacitor 20 becomes large, so that the diode 21 can not be switched at high frequency. The clamp circuit performs a clamp operation at a fixed voltage as defined by the peak voltage of the energy dispersal signal at low frequency. Therefore, this clamping operation can not remove the energy dispersal signal and also can not clamp the video signal. Conversely, when the capacitance of the capacitor 20 is selected to be small, the charge/discharge time constant of the capacitor 20 becomes small. In this case, when the horizontal and vertical synchronizing pulse signals are applied to the input terminal 19, the diode 21 is conductive to set at V.sub.0 the peak voltage of the synchronizing pulse superposed on the video signal. As a result, the energy dispersal signal 16 is suppressed and the video signal 17 is clamped at voltage Vo, as shown in FIG. 4. The video signal 17 thus clamped is output from the output terminal 25. Now, a defect of the conventional diode clamp circuit is that a sag 26 occurs in the vertical synchronizing signal because the vertical synchronizing signal 18 renders the diode 21 imperfectly conductive. If this sag 26 is large, the synchronization in the television receiver is performed incorrectly.
Recently in the U.S.A., a pay television system has become prevalent. In order to prevent the theft of service, this pay TV system employs a system of using a scrambled signal in which the television signal is modified or scrambled so as not to be received by the normal TV set unless it is provided with an adaptor specially designed so as to receive the scrambled signal. A typical example of this scrambling system is disclosed in a paper entitled PAY-TV DECODER in "RADIO-ELECTRONICS", February 1981, pp 41-54. In this system, the horizontal and vertical synchronizing signals are suppressed, but the synchronizing information instead of the suppressed synchronizing pulse is contained in the audio signal. For this reason, the normal TV set when receiving the scrambled signal will not reproduce a coherent picture on the screen due to the asynchronism of the TV receiver with the TV broadcasting station. In executing the scrambling system using the waves in SHF band relayed by the artificial satellite, the energy dispersal signal must additionally be contained in the television signal. To reproduce such television signal, the energy dispersal signal must completely be removed from the television signal and then subjected to various signal processings. This scrambling system suppreses the synchronizing signal, as described above. Therefore, it is evident that the energy dispersal signal can not be removed by using the diode clamp circuit of the type in which the synchronizing signal is used for removing the energy dispersal signal.